The present invention relates to a loading probe for testing an acoustic transducer at ultrasonic frequencies, which is particularly, but not exclusively, suitable for testing an underwater acoustic transducer in air.
Transducers such as piezoelectric transducers may be tested by simulating the normal acoustic impedance presented to them in the medium, such as a liquid, for example water, in which they are intended to be operated. Such transducers are intended to generate or detect acoustic signals.
GB-B-2259427 and GB-B-2217952 both describe loading probes which are for the testing in air of electro acoustic transducers and which have been used successfully at frequencies in the single figure kHz range. At such low frequencies high power requirements can be satisfied with relatively low power density (flux) values because of the relatively large dimensions of the probe at such frequencies where a typical power density (flux) would be of the order of 0.5 W per mm2 cross-sectional area. Such known loading probes can also be used at ultrasonic frequencies of tens of kHz provided the power density (flux) levels are kept to a maximum level of 0.5 W per mm2 cross-sectional area. Because of the reduction in wavelength and consequent dimensions, however this restricts operation at ultrasonic frequencies to low power levels, whereas for similar high power applications power density (flux) requirements are significantly higher in the order of 5.0 W per mm2 cross-sectional area. Whilst the loading probes of GB-B-2217952 and GB-B-2259427 are generally successful they are not suitable for high power testing at ultrasonic frequencies partly by virtue of their design and construction and partly by virtue of the materials from which they are made. For example at power densities (flux) of the order of 5.0 W per mm2 cross-sectional area the material, which is generally an acrylic polymer from which the probe is made is heated and at temperatures in the range of 30xc2x0 C. to 40xc2x0 C. the acoustic absorption increases by the onset of beta-relaxation which leads to thermal runaway so that the material rapidly softens as it reaches glass transition temperature. This not only significantly alters the acoustic absorption characteristics but detrimentally reduces the working life of the probe.
Additionally the construction of such known probes is such that the main body of the probe is hollow and contains an acoustically absorbent material which is usually a partially polymerised metal loaded epoxy resin. At ultrasonic frequencies this material has an undesirably high acoustic loss factor leading to rapid heating of the resin over a short length at the beginning of the volume formed by the loading probe tube so that at elevated temperatures above room temperature the resin acoustically absorbent material starts to age as it Is not a stable system. At higher temperatures the material can become liquid and age rapidly whereupon the material hardens and becomes discontinuous with the remaining material of the probe. This causes reflections and renders the probe unsuitable for further use.
There is thus a need for a generally improved loading probe which is able to operate at ultrasonic frequencies without degradation of material properties as may be experienced by the conventional loading probes of GB-B-2217952 and GB-B2259427 as described above.
According to a first aspect of the present invention there is provided a loading probe for testing an acoustic transducer, including an elongated solid first body portion for carrying a piezo-electric test element, which elongated solid first body portion is shaped from a first material and capable of simulating the normal acoustic impedance presented to a transducer to be tested by a medium in which the transducer is to operate, and an elongated solid second body portion made from an acoustically absorbent material having acoustic impedance characteristics substantially matched with those of the first material from which the first body portion is made, with the second body portion being attached at one end to one end of the first body portion, characterised in that the second body portion is substantially rod like in shape and the material from which the second body portion is made is an elastomeric polyurethane having an absorption coefficient higher than that of said first material over a temperature range of 5xc2x0 C. to 130xc2x0 C.
Preferably the elastomeric polyurethane material contains silicon dioxide, silicon nitride, tungsten powder, tungsten carbide, tungsten boride, tungsten silicide, molybdenum powder, molybdenum carbide, molybdenum nitride, tantalum powder, tantalum carbide or tantalum nitride in an amount chosen to match the acoustic impedance of the second body portion material to that of the first body portion material.
Conveniently the second body portion is made from discrete stages or layers of elastomeric polyurethane material, with each stage or layer having differing properties sequentially along the second body portion.
Alternatively the second body portion is made from continuously graded elastomeric polyurethane material.
Conveniently the first material from which the first body portion is made is a polycarbonate, polyethylene terephthalate or polyethersulphone having a low absorption coefficient over a temperature range of 5xc2x0 C. to 200xc2x0 C.
Conveniently the first and second body portions are attached to one another by a projection provided at one end of one body portion engaging in a correspondingly shaped recess provided in one end of the other body portion.
Advantageously the projection and recess are conical in shape.
Preferably the second body portion utilised is formed by moulding.
Conveniently the loading probe is constructed and dimensioned to simulate the operating medium.
Advantageously the loading probe is operable to test an acoustic transducer in air.
Preferably the first body portion is substantially circular in cross section having a waisted profile formed by two intersecting opposed frusto-conical sections, dimensioned to assist matching of the acoustical impedance of the first body portion to that of the medium in which the transducer to be tested is to operate.
Advantageously the piezoelectric test element is carried by the first body portion intermediate the ends of the first body or at the end of the first body portion opposite to said one end thereof.
According to a second aspect of the present invention, there is provided measurement apparatus comprising two loading probes as described above arranged end to end with material to be measured sandwiched therebetween.